jtag/drivers/ti_icdi: do not segfault when adapter can't be opened

Change-Id: Id3af8dfd18b13947bca4f3c89c2516ccbcef60b6
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2742
Tested-by: jenkins

src/jtag/drivers/ti_icdi_usb.c

@@ -654,6 +654,9 @@ static int icdi_usb_close(void *handle)
 {
 	struct icdi_usb_handle_s *h = handle;
 
+	if (!h)
+		return ERROR_OK;
+
 	if (h->usb_dev)
 		libusb_close(h->usb_dev);